1. Field of the Invention
The present invention relates generally to thin films and methods for their deposition, and more particularly, provides diamond-like films, plasma beam deposition systems, and methods useful for production of diamond-like protective overcoats on magnetic recording media and other industrial applications.
In recent years, there has been considerable interest in the deposition of a group of materials referred to as diamond-like carbon. Diamond-like carbon can generally be defined as a metastable, high density form of amorphous carbon. Diamond-like carbon is valued for its high mechanical hardness, low friction, optical transparency, and chemical inertness.
Deposition of diamond-like carbon films often involves chemical vapor deposition techniques, the deposition processes often being plasma enhanced. Known diamond-like films often include carbon with hydrogen, fluorine, or some other agent. The durability and advantageous electrical properties of diamond-like carbon films have led to numerous proposals to apply these films to semiconductors, optics, and a wide variety of other industrial uses. Unfortunately, the cost and complexity of providing these advantageous diamond-like carbon films using known chemical vapor deposition processes has somewhat limited their use. Furthermore, while a wide variety of diamond-like carbon coating films have been deposited in laboratories, many of these films have been found to have less than ideal material characteristics.
A very different form of amorphous carbon is generally applied as a protective overcoat for magnetic recording media. Magnetic recording disks generally comprise a substrate having a magnetic layer and a number of underlayers and overlayers deposited thereon. The nature and composition of each layer is selected to provide the desired magnetic recording characteristics, as is generally recognized in the industry.
The information stored in magnetic recording media generally comprises variations in the magnetic field of a thin film of ferromagnetic material, such as a magnetic oxide or magnetic alloy. Usually, a protective layer is formed over the top of the magnetic layer, and a layer of lubricating material is deposited over the protective layer. These protective and lubricating layers combine to increase the reliability and durability of the magnetic recording media by limiting friction and erosion of the magnetic recording layer. Sputtered amorphous carbon films have gained widespread usage as protective overcoats for rigid magnetic recording disks.
Sputtered amorphous carbon overcoats have been shown to provide a high degree of wear protection with a relatively thin protective layer. Magnetic recording disk structures including sputtered amorphous carbon have been very successful and allow for quite high recording densities. As with all successes, however, it is presently desired to provide magnetic recording disks having even higher recording densities.
Recording densities can generally be improved by reducing the spacing between the recording transducer, called the read/write head, and the magnetic layer of the magnetic recording disk (or more specifically, between the read/write head and the middle of the magnetic layer). In modern magnetic recording systems, the read/write head often glides over the recording surface on an air bearing, a layer of air which moves with the rotating disk. To minimize frictional contact between the rotating disk and the read/write head, the disks surface is generally rougher (and the glide height therefore higher) than would otherwise be ideal for high density magnetic recording. Even if this glide height is reduced (or eliminated), the read/write head will be separated from the recording layer by the protective amorphous carbon overcoat. This protective layer alone may, to provide the desired media life, limit the areal density of the media. Generally, overcoat layer thicknesses are dictated by durability and continuity limitations. Sputtered carbon frequently becomes discontinuous at thicknesses below about 50 xc3x85. Thus, the durability requirements of rigid magnetic recording media generally dictate that the distance between the read/write head and the magnetic recording layer be maintained, even though this limits the areal density of the magnetic recording media.
It has previously been proposed to utilize known chemical vapor deposition techniques to deposit a variety of diamond-like carbon materials for use as protective coatings for flexible magnetic recording tapes and magnetic recording heads. Unfortunately, known methods for chemical vapor deposition of diamond-like materials, including plasma enhanced methods, generally subject the substrate to temperatures of over 500xc2x0 C., which is deleterious for most magnetic disk substrates. Therefore, these known diamond-like carbon films do provide relatively good hardness and frictional properties, they have found little practical application within the field of rigid magnetic recording media, in which sputtered amorphous carbon protective overcoats are overwhelmingly dominant.
For these reasons, it would be beneficial to provide improved magnetic protective overcoats with improved read/write head frictional and glide characteristics (generally called stiction) for recording media. Preferably, such an improved overcoat will provide durability and reliability without having to resort to the density-limiting glide heights and/or protective overcoat thickness of known rigid magnetic recording media, and without subjecting the media substrates to excessive temperatures.
It would also be desirable to provide improved diamond-like carbon materials and methods for their deposition. It would be particularly desirable if such materials and methods could be utilized for practical rigid magnetic recording media with reduced spacing between the read/write head and the magnetic recording layer, ideally by providing a flatter, smoother, and thinner protective coating which maintained or even enhanced the durability of the total recording media structure. It would also be advantageous to provide alternative methods and systems for depositing such protective layers, for use in the production of magnetic recording media, as well as integrated circuits, optics, machine tools, and a wide variety of additional industrial applications.
2. Description of the Background Art
U.S. Pat. No. 5,182,132 describes magnetic recording media having a diamond-like carbon film deposited with alternating circuit plasma enhanced chemical vapor deposition methods. U.S. Pat. No. 5,462,784 describes a fluorinated diamond-like carbon protective coating for magnetic recording media devices. European Patent Application 700,033 describes a side-mounted thin film magnetic head having a protective layer of diamond-like carbon. European Patent Application No. 595,564 describes a magnetic recording media having a diamond-like protective film which consists of carbon and hydrogen.
U.S. Pat. No. 5,156,703 describes a method for the surface treatment of semiconductors by particle bombardment, the method making use of a capacitively coupled extraction grid to produce an electrically neutral stream of plasma. V. S. Veerasamy et al. described the properties of tetrahedral amorphous carbon deposited with a filtered cathodic vacuum arc in Solid-State Electronics, vol. 37, pp. 319-326 (1994). The recent progress in filtered vacuum arc deposition was reviewed by R. L. Boxman in a paper presented at the International Conference of Metallurgical Coatings and Thin Films located at San Diego in April of 1996. Electron cyclotron wave resonances in low pressure plasmas with a superimposed static magnetic field were described by Professor Oechsner in Plasma Physics, vol. 15, pp. 835-844 (1974).
The present invention provides systems and methods for the deposition of an improved diamond-like carbon material, particularly for the production of magnetic recording media. The diamond-like carbon material of the present invention is highly tetrahedral, that is, it features a large number of the sp3 carbon-carbon bonds which are found within a diamond crystal lattice. The material is also amorphous, providing a combination of short-range order with long-range disorder, and can be deposited as films which are ultrasmooth and continuous (pin-hole free) at thicknesses substantially lower than known amorphous carbon coating materials. The carbon protective coatings of the present invention will often be hydrogenated, generally providing a significantly higher percentage of carbon carbon sp3 bonds than known hydrogenated amorphous diamond-like carbon coatings having similar compositions, and may optionally be nitrogenated. In a preferred method for depositing of these materials, capacitive coupling forms a highly uniform, selectively energized stream of ions from a dense, inductively ionized plasma. Such inductive ionization is enhanced by a relatively slow moving (or xe2x80x9cquasi-staticxe2x80x9d) magnetic field, which promotes resonant ionization and ion beam homogenization. Clearly, the materials, systems, and methods of the present invention will find applications not only in the field of magnetic recording media and related devices, but also in integrated circuit fabrication, optics, machine tool coatings, and a wide variety of film deposition and etching applications.
In a first aspect, the present invention provides a method for producing magnetic recording media, the method comprising forming a magnetic layer over a substrate, and ionizing a source material so as to form a plasma containing carbon ions. The carbon ions are energized to form a stream from the plasma toward the substrate, so that carbon from the ions is deposited on the substrate. The ions impact with an energy which promotes formation of sp3 carbon-carbon bonds. Advantageously, such a method can form a highly tetrahedral amorphous carbon protective layer, generally having more than about 15% sp3 carbon-carbon bonds. Generally, the impact energy of the energetic carbon ions is within a pre-determined range to promote formation of the desired lattice structure, the bonds apparently being formed at least in part by subplantation. Preferably, each carbon ion impacts with an energy of between about 100 and 120 eV. In many embodiments, the resulting highly tetrahedral amorphous carbon protective layer includes more than about 35% sp3 carbon-carbon bonds, with particularly preferred methods producing more than about 70% sp3 carbon-carbon bonds.
Generally, the stream will be primarily composed of ions having a uniform weight, and the impact energy will preferably be substantially uniform. In some embodiments, this uniformity is promoted through filtering of the ion stream. In such cases, the energizing step generally comprises striking a plasma using a solid cathodic arc of carbon source material. Alternatively, the stream will be energized by applying an alternating potential between a coupling electrode and an extraction grid so as to self-bias the plasma relative to the extraction grid through capacitive coupling, thereby extracting the ion stream through the grid. Hydrogen and/or nitrogen may also be included, both in the ion stream and the protective layer.
In another aspect, the present invention provides magnetic recording media comprising a substrate, a magnetic layer disposed over the substrate, and a protective layer disposed over the magnetic layer. The protective layer comprises a highly tetrahedral amorphous carbon, generally having more than about 15% sp3 carbon-carbon bonds. Preferably, these bonds are formed at least in part by directing an energetic stream of carbon ions onto the substrate. In many embodiments, the protective layer includes more than about 35% sp3 carbon-carbon bonds, with particularly preferred embodiments including more than about 70% sp3 carbon-carbon bonds. Such protective layers are ultrasmooth and continuous at thicknesses of less than about 75 xc3x85, and will provide durable recording media even at thicknesses of less than about 50 xc3x85. Furthermore, the hardness and tribological performance of these dense protective materials may allow highly durable recording media with areal recording densities of over 1 gigabyte per square inch with reduced read/write head glide heights of lower than about 1xcexcxe2x80x3, optionally within a near-contact or continuous contact recording systems.
In another aspect, the present invention provides a method for enhancing an ion beam, the ion beam produced by confining a plasma within a plasma volume, inductively ionizing the plasma, and forming a stream of ions from within the plasma volume by capacitive coupling. The method comprises moving a magnetic field through the plasma to promote resonant inductive ionization, preferably by sequentially energizing each of a plurality of coils disposed radially about the plasma volume.
In another aspect, the present invention provides an inductive ionization system for use with an ion-beam source. The source includes an antenna disposed about a plasma volume for inductively ionizing a plasma therein. A coupling electrode is exposed to the plasma volume and an extraction electrode is disposed over an opening of the plasma volume, so that the extraction electrode is capable of expelling ions of the plasma through the grid by capacitive coupling. The system comprises at least one coil disposed adjacent the plasma volume capable of applying a transverse magnetic field to the plasma volume so as to promote resonant inductive ionization by the antenna. The magnetic field can be moved through the plasma container to homogenize the expelled ion stream. This movement of the magnetic field, which is optionally provided by selectively energizing coils radially disposed about the plasma volume, may also further densify the plasma by promoting particle collisions through a churning or mixing effect.
In another aspect, the present invention provides a diamond-like material comprising carbon in the range between about 72 and 92 atomic percent, and hydrogen in the range between about 8 and 18 atomic percent. The material is amorphous, and between about 15 and 85 percent of carbon-carbon bonds are sp3 bonds. Generally, sp3 bond formation will be promoted with subplantation using ion-beam deposition from a plasma beam source, so that the number of such bonds will be higher than known materials having similar compositions. Hence, the highly tetrahedral amorphous carbon and hydrogenated carbon of the present invention will have fewer polymer-like hydrogen chains, and will generally exhibit enhanced thermal and mechanical stability.
In another aspect, the present invention provides a method for deposition of highly tetrahedral amorphous carbon over a substrate, the method comprising ionizing a source material to form a plasma and confining the plasma within a plasma volume. The plasma is capacitively coupled to form a stream flowing outwardly from within the plasma volume. The stream includes carbon ions from the plasma and is directed onto the substrate. Advantageously, such a method allows deposition of carbon ions of uniform size with a uniform energy, and allows tailoring of the energetic carbon ions to specifically promote sp3 bonding through subplantation. The source material typically comprises a gas having a substantially coherent dissociation energy spectra, the source gas ideally comprising acetylene. Preferably, the ions strike the substrate with an impact energy of between about 57 and 130 eV for each carbon atom, ideally being between about 80 and 120 eV each.
In another aspect, the present invention provides an ion-beam source comprising a container defining a plasma confinement volume. The container has an opening, and an antenna is disposed about the plasma volume so that application of a first alternating potential to the antenna is capable of inductively ionizing a plasma therein. A coupling electrode is electrically coupled to the plasma volume and an extraction electrode is disposed over the opening of the container. The extraction electrode has a surface area which is substantially less than the coupling electrode surface, so that application of a second alternating potential between the coupling electrode and the extraction electrode is capable of expelling ions of the plasma through the grid. Preferably, at least one coil is disposed adjacent the container, and is capable of applying a transverse magnetic field to the plasma volume, thereby promoting highly efficient inductive ionization resonance by the antenna. Ideally, the magnetic field can be moved through the plasma container to homogenize the expelled ion stream. This movement of the magnetic field, which is optionally provided by selectively energizing coils radially disposed about the plasma confinement volume, may further density the plasma by promoting particle collisions with a churning or mixing effect.
In yet another aspect, the present invention provides an ion-beam source comprising plasma containment means for confining a plasma within a plasma volume. Inductive ionization means inductively couples a first alternating current with the plasma so as to ionize the plasma within the plasma volume. A moving magnetic field generation means provides resonant densification and homogenization of the ionized plasma within the plasma volume. Ion extraction means forms a stream of ions out from the plasma volume.
In another aspect, the present invention provides a method for producing an ion beam, the method comprising confining a plasma within a plasma volume, inductively ionizing the plasma, and forming a stream of ions from within the plasma volume by capacitively coupling the plasma with an extraction grid. This capacitive coupling self-biases the plasma relative to the grid, and can be used to produce a quasi-neutral plasma stream. Generally, a transverse magnetic field is applied to density the plasma by promoting resonant inductive ionization. Ideally, the magnetic field is moved through the plasma volume to homogenize the plasma and plasma stream.